1. Field of the Invention
The present invention relates to a hydrodynamic bearing device employing a hydrodynamic bearing and a motor including such a hydrodynamic bearing device.
2. Description of the Related Art
More and more hydrodynamic bearing devices are replacing ball bearing devices which have been conventionally used in spindle motors in hard discs, polygon mirrors, optical disc apparatuses and the like. The hydrodynamic bearing devices are superior to the ball bearings in a rotational accuracy and silent property.
More and more hard discs and the like which employ such hydrodynamic bearing devices are being used in mobile computing devices such as portable computers. Thus, hydrodynamic bearing devices have to have a high reliability over a wide range of an operating temperature. In the hydrodynamic bearing devices used in such devices, lubricating oil used in bearings has a property that its viscosity varies depending upon the temperature. This leads to a drawback in a temperature property. Specifically, a current consumption becomes large as the viscosity of the lubricating oil is increased at a low temperature, and a rigidity of the lubricating oil is reduced as the viscosity of the lubricating oil is decreased at a high temperature. In order to improve such a temperature property, a method to equalize bearing clearances at a high temperature and a low temperature is used.
For equalizing bearing clearances at a high temperature and a low temperature, for example, a coefficient of linear expansion of a shaft material has to be equal to that of a sleeve material in a shaft in a hydrodynamic bearing device and a sleeve which supports the shaft so as to be rotatable. Therefore, it has been considered that, when the sleeve is formed using a copper metal material having a large coefficient of linear expansion, an austenite stainless steel such as SUS303, which has a coefficient of linear expansion larger than those of martensite stainless steels such as SUS420J2, SUS440C, and the like which have been conventionally used for a shaft, is used. However, when the shaft is formed of an austenite stainless steel and is used, abrasion of a bearing surface is accelerated since its surface hardness is small compared to those of martensite stainless steels which are usually used. Abrasion of the bearing surface may result in an abnormal rotation, or even in a locked state of the bearing in which a rotation of the shaft stops at worst. Therefore, when an austenite stainless steel is used for a shaft as it is, there is a problem that the reliability of the bearing device deteriorates significantly.
In order to solve this problem, Japanese Laid-Open Publication No. 10-089345 discloses a hydrodynamic bearing device in which a shaft member formed of an austenite stainless steel is treated with nitriding for improving its surface hardness. For example, one of austenite stainless steels, SUS303, is formed into a desired shape by a cutting process, and a dynamic pressure groove is formed by plastic working. Then, a finishing process is performed by polishing. Further, a surface hardening process is performed by nitriding such as: 1) salt bath nitriding; 2) ion nitriding; 3) gas nitrocarburizing; and the like to obtain the shaft.
Japanese Laid-Open Publication No. 2000-297813 discloses a hydrodynamic bearing device in which a cold working rate for a bearing member formed of an austenite stainless steel is 20% or higher, thereby providing a surface hardness of 300 Hv or higher. For example, a steel ingot is processed with hot rolling (bar and wire-rod rolling). Then, cold working with a cold working rate (reduction of area) of 20% or higher by cold rolling or cold drawing. The obtained austenite stainless steel having the surface hardness of 300 Hv or higher is used to form the shaft.
However, the conventional hydrodynamic bearing device disclosed in Japanese Laid-Open Publication No. 10-089345 suffers from a surface roughness due to nitriding. When the stainless is used as it is after nitriding, there are problems such as contamination due to insufficient cleaning or acceleration of abrasion of the bearing surface. If the surface is polished again after nitriding, the surface roughness may be alleviated. However, this method requires an additional process step, and results in an increase in costs.
The conventional hydrodynamic bearing device disclosed in Japanese Laid-Open Publication No. 2000-297813 has the problem that a variance in performance such as hardness is large since the cold working rate is raised to 20% or higher in order to improve the hardness. Thus, it is difficult to secure a certain performance. This causes that the hardness and constitution vary depending upon the site to be cut. Thus, a machinable property deteriorates, and thus, a working accuracy deteriorates.
In the conventional hydrodynamic bearing devices disclosed in Japanese Laid-Open Publication Nos. 10-089345 and 2000-297813, the hardness of the surfaces of the shafts are improved. Thus, it is possible to prevent abrasion of the shafts. However, since copper metal materials having small hardness are used for the sleeves which hold the shafts so as to be rotatable, there is a problem that the sleeves are worn away. Since copper contained in the copper metal materials has a function to accelerate deterioration of lubricating oil which is a working fluid used in hydrodynamic bearings, the reliability of the hydrodynamic bearing device deteriorates.
In order to solve the above-described problems in the conventional hydrodynamic bearing devices, the present invention provides a hydrodynamic bearing device having a high performance and reliability which prevents deterioration in a temperature property due to a difference in coefficients of linear expansions of a sleeve and a shaft, and prevents abrasion of bearing surfaces of both the sleeve and the shaft, and a motor including such a hydrodynamic bearing device.